Printing method and article for hiding halftone images

ABSTRACT

A method of halftone printing is disclosed which permits an image to be disguised or hidden in a printed field from detection by ordinary photography or the eye of a viewer and an apparatus is presented which can be used to bring out or detect the hidden image. The printed field comprises three halftone components: an overall halftone at a given angle; a second halftone at a different angle and containing a negative of the image; and the image itself in halftone at a third angle and coincident with the negative portion of the second halftone. When each of the halftones is made up of dot figures of similar size, period and tone, the field will give a uniform appearance to the eye, obscuring the image and resisting photographic analysis except by impractical magnification. An apparatus in the form of a screen made up of a periodic pattern of similar frequency to that of the halftone screen of the printed field and arranged at the proper angle to the printed field, will reveal the hidden image at a different brightness than the background. By moving the properly arranged screen in an appropriate manner relative to the printed field, the hidden image will appear as a blinking image. If the method and apparatus are used in printing and checking security documents, additional printing techniques are disclosed for preventing counterfeiting of the documents such as black overprinting, slight variations in the print angle from the conventional angles and the use of unique and complicated dot figures and images.

United States Patent Wicker [54] PRINTING METHOD AND ARTICLE FOR HIDINGHALFTONE IMAGES [72] Inventor: Ralph C. Wicker, Fairport, NY.

[73] Assignee: American Bank Note Company, New

York, NY.

[22] Filed: Sept. 10, 1969 [21] App1 No.: 856,663

1,928,758 10/1933 Mairson et a]. ....10l/21l 2,030,351 2/1936 Clifford....101/21l 2,048,780 7/1936 Clifford 101/211 2,952,080 9/1960 Avakianet a1. ..283/6 3,279,095 10/1966 Carlson ..35/2 3,288,059 11/1966Atkinson ....l01/2ll 3,381,612 5/1968 Lecha ..101/211 X PrimalExaminer.l. Reed Fisher Attorney-Robert S. Dunham, P. E. Henninger,Lester W. Clark, Gerald W. Griffin, Thomas F. Moran, Howard J.

[4 1 July 11, 1972 [5 7] ABSTRACT A method of halftone printing isdisclosed which permits an image to be disguised or hidden in a printedfield from detection by ordinary photography or the eye of a viewer andan apparatus is presented which can be used to bring out or detect thehidden image. The printed field comprises three halftone components: anoverall halftone at a given angle; a second halftone at a differentangle and containing a negative of the image; and the image itself inhalftone at a third angle and coincident with the negative portion ofthe second halftone. When each of the halftones is made up of dotfigures of similar size, period and tone, the field will give a uniformappearance to the eye, obscuring the image and resisting photographicanalysis except by impractical magnification.

An apparatus in the form of a screen made up of a periodic pattern ofsimilar frequency to that of the halftone screen of the printed fieldand arranged at the proper angle to the printed field, will reveal thehidden image at a different brightness than the background. By movingthe properly arranged screen in an appropriate manner relative to theprinted field, the hidden image will appear as a blinking image. If themethod and apparatus are used in printing and checking securitydocuments, additional printing techniques are disclosed for preventingcounterfeiting of the documents such as black overprinting, slightvariations in the print angle from the conventional angles and the useof unique and complicated dot figures and images.

13 Claims, 9 Drawing Figures SHEET 10! 5 PATENTEDJUL 1 1 I972 Rm m N W m7 m0 4 W Wm,

PRINTING METHOD AND ARTICLE FOR HIDIN HALFIONE IMAGES BACKGROUND OF THEINVENTION The present invention relates to the printing or lithographicart and more particularly to a method and apparatus for producing andviewing an image hidden in a halftone field.

The techniques of halftone printing are well known in the printing art.They consist essentially of photographing the picture or figure to bereproduced, through a screen and using the screened photograph to etchthe printing plate. The printing surface of the plate thus contains thepicture to be reproduced in the form of a pattern of dot figures whichcorrespond to the dot figures in the screen and which may be in the fonnof circles, squares, triangles or the like. The printed picture is thencomposed of corresponding regularly arranged lines of dot figures and isreferred to as a halftone. To achieve different effects in the printedpicture, variations may be made in the density of the screen, that is,the dot size and spacing; the color and tone of the ink, and the angleat which the lines of dots are arrayed. A single picture may be producedusing more than one halftone pattern by arranging the componenthalftones at different angles.

It has been found in printing the various components in a halftonepicture at different angles, that certain light interference effectscalled moire efiects, may occur which cause waviness, blotching andother distortions in the printed picture. As a result of thisphenomenon, it has become conventional to print the various componentsin a halftone picture at different angles of approximately 30displacement from each other so that, for example, in multi-colorprinting, the red component of the picture may be printed at 45, theyellow component at 75 and the blue component at 105 with respect to theupper margin.

I-Ialftone printing is widely used and is the technique primarily usedin multicolor reproduction and for producing the patterns and figures onsecurity documents, such as currency, bonds, stock certificates and thelike. While documents printed in this manner are somewhat difficult toduplicate or counterfeit, still in the past few techniques have beenfound to prevent those skilled in the art from producing counterfeitswhich resist being readily distinguishable or identifiable from theoriginal documents.

The present invention provides a method of halftone printing whichpermits a distinctive image to be hidden in a halftone field in suchmanner as to prevent detection by the unaided eye or ordinaryphotographic techniques, and also provides a suitable apparatus whichmay be used to quickly bring out the hidden image. The invention hasmany diverse applications and is particularly useful in the securitydocuments field as it renders documents so printed resistant tocounterfeiting and permits ready identification of valid documents anddetection of invalid duplicates.

SUMMARY OF THE INVENTION The printing method of the present inventioncomprises the production of a halftone field composed of: an overallhalftone printed at a given angle; a second halftone printed at adifferent angle and containing a negative of the image to be hidden; andthe image itself printed in halftone at a third angle and coincidentwith the negative portion of the second halftone.

The three halftone components are made up of dot figures having similarfrequencies so that a printed field or rosette pattern is formed whichgives a uniform appearance to the eye, obscuring the image. Usingordinary halfione frequencies such as 100 or 150 dot figures to theinch, the hidden image also will resist photographic analysis as theoptical magnification required to distinguish the various componentswould be impractical under most circumstances. The image may be readilydetected, however, by the use of the apparatus of the present inventionwhich comprises a screen made up of a periodic pattern of similarfrequency as the halftones and capable of being arranged at the sameangle as the halftones. When such a screen is arranged at the angle ofthe halftone screen of the hidden image, the hidden image can be made toappear as a bright image against a darker background. Alternatively,when the screen is arranged at the same angle as the halftone screen ofthe background, the hidden image can be made to appear as a darker imageagainst a brighter background. At either orientation when the screen ismoved parallel to the periodic pattern the brighter image will appear toblink.

To further confound attempts at duplication or counterfeiting ofarticles and documents printed in this manner, even using the apparatusof the invention, a black overprint may be added to the three componentsin the halftone field and the print angle of the three components may bevaried somewhat from the conventional thirty degree displacement withoutcausing objectionable moire efiects. Also, a solid over printing colorof the same reflection density as the printed document may be used. Inaddition, unusual dot configurations may be used and the hidden imagemay be of a unique or complicated form. Also, a like color can be overprinted at a different angle and/or a different frequency to present aserious moire problem to the potential counterfeiter.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a light table with asheet of film disposed thereon containing a halftone arranged at anangle of 45 with respect to the upper margin, and which is used inproducing one component of the printed field of the present invention;

FIG. la is an enlarged view of a portion of the film of FIG. 1 showingthe halftone in detail;

FIG. 2 shows two sheets of film disposed on the light table, the upperone being transparent except for the opaque characters OK and the lowerone containing a halftone arranged at an angle of 75 with the uppermargin, and which provides another component of the printed field of thepresent inventron;

FIG. 3 shows another two sheets of film on the light table, the upperone being opaque except for a transparent portion in the form of thecharacters OK and the lower one containing a halftone arranged at anangle of 105 with the upper margin and which provides a furthercomponent of the field of the present invention;

FIG. 4 shows an article printed with a representation of the compositehalftone field of the present invention;

FIG. 5 is a perspective view of an apparatus which may be used to viewthe hidden image on a document printed in the manner of the presentinvention.

FIG. 6 shows a representation of the hidden image on a document beingviewed by the apparatus;

FIG. 7 shows the document of FIG. 6 printed with an additional blackover print arranged at with respect to the upper margin; and

FIGS. 8 and 9 are modifications of the apparatus employing surfaceillumination of the document.

DETAILED DESCRIPTION OF THE INVENTION The basic technique of producing ahalftone print consists of photographing the picture or figure to bereproduced through a screen and using the screened photograph to etchthe surface of the printing plate. The printing surface of the platethus contains the picture to be reproduced in the form of a pattern ofdot figures which correspond to the dot figures in the screen. Thedifferent components of a composite halftone print are produced bysuccessively photographing the picture with the screen arranged atdifferent angles. If the picture is to be printed in a single color, allof the components may be etched, using the succession of photographs, ona single plate and when a multicolor reproduction is made, eachphotograph is used to etch an individual corresponding plate. The methodof the present invention will first be described in connection withprinting a halfione field of a single color and then in connection witha multicolor field. While certain particular steps will be described forperforming the method of the present invention, since the photographicart lends itself to the use of various combinations of negatives andpositives in different sequences to produce the same ultimate result,those skilled in the art will see numerous equivalent latemative stepswhich may be used in performing the method of the present invention.

HIDDEN IMAGE Firstly, a large sheet of commercially available film isobtained containing a negative or a positive pattern of a halftonescreen. For example, the screen pattern may be composed of square 'dotfigures having a density of 40 percent and a frequency of 100 per inch,that is, the film will contain a positive of the screen, the dots beingtransparent, and the opaque areas in between them covering 40 percent ofthe total area of the film. It should be understood in this regard thatthe screen patterns shown in the drawings are illustrativerepresentations to be used in explaining the invention and thereforelack the exact physical properties and visual qualities of the screensand printed halftones which are actually used, since it is the intentionof the invention that in actuality the halftones obscure analysis. Thedrawings then are intended to provide some impression of the actualvisual effects while still aiding in the understanding of the invention.

Now, in the first step, a section is cut from the large sheet of film inthe shape of, but slightly larger than the size of the printing plate tobe produced, such as in the form of the rectangular sheet 1 shown inFIG. 1. The lines of dots 2, of the screen pattern are arranged at anangle of 45 with the upper margin as illustrated in detail in FIG. la.Two registration holes 3 may be punched near the upper margin in theportion of the cutout sheet 1 which is in excess of the size of theprinting plate. The cutout sheet 1 is then placed on a light table 4 asshown in FIG. 1, registered, and a reverse picture is made by exposing asheet of film in a contact vacuum frame 5. The cutout sheet 1 is thenremoved from the light table 3 in preparation for the second step.

A second section is cut from the film in the shape and size of the firstcutout sheet I, but with the lines of dots 2 on the screen arranged atan angle of 75 with the upper margin (FIG. 2). Two registration holes 3aare again punched near the upper margin of this cutout sheet and thesheet 6 is placed on the light table 4 with the registration holes 3apositioned in coincidence with the positioning of the registration holes3 of the first sheet I.

A sheet of transparent film 7 is now obtained in the shape of the firsttwo cutout sheets, I and 6, and registration holes 3a are punched inthis film. This sheet of film 7, contains a positive pattern of theimage which is to be hidden. For our purposes, the image will be in theform of the letters OK as shown in FIG. 2. The transparent sheet of film7 is then placed on the light table 4 on top of and in registration withthe second cutout sheet 6 and a second exposure of the film in thecontact vacuum frame 5 is made. The film in the contact vacuum frame 5will now contain coincident negatives of the overall 45 halftone and the75 halftone with a blank of the letters OK in the latter negative. Thetwo sheets of film 6 and 7 are then removed from the light table 4 inpreparation for the third step.

A third section is cut from a large sheet of film in the shape and sizeof the first two cutout sheets, 1 and 6, but having the lines of dots 2of the screen arranged at an angle of 105 with the upper margin. Theregistration holes 3b are punched in the upper margin and the sheet 8 isplaced in registration on the light table 4 as shown in FIG. 3.

A sheet of film 9 which is the reverse of the transparent sheet 7 usedin the second step, that is, opaque, except for a transparent portion inthe image OK, is then obtained and registration holes 3b punched in itsupper margin. To check that this film 9 is a perfect reversal of sheet7, the sheet 7 may be placed in registration on the light table 4 andthe reversal sheet 9 placed in registration over it. The combination ofthe two sheets 7 and 9, should provide a completely opaque field on thelight table 4. After checking, the transparent sheet 7 is removed andthe reversal sheet 9 is placed over the sheet 8 which is screened atI05". A third exposure is then made of the film in the contact vacuumframe 5, which will impress a negative of the screen only in the areasof the image OK. The 105 screen image will occur exactly within thenegative areas of the image produced during the second exposure.

The film containing the composite negative of the three screen patternsis then used to etch a printing plate and a representation of thehalftone field l0 printed on an article 11 with the plate is shown inFIG. 4. The combination of the three screen patterns printed atdifferent angles produces what appears to the eye to be a uniformhalftone field 10. The image of the letters OK screened at 105, will behidden from the eye in the uniform field 10. At a halftone period of I00or dots per inch, (much greater than that shown in FIG. 4) the imagewill defy analysis by ordinary magnification methods. While it isconceivable that the halftone could be enlarged to the point whereanalysis would be possible, such analysis would be completelyimpractical with existing equipment.

Using a single plate, the halftone field 10 must, of course, be printedin one color. However, images may also be hidden in this manner inmulti-color halftone fields as long as there is no great difference inthe density and tone of the various screen pattern components. Forexample, a field printed with red, green and yellow components ofsimilar tone and each of a density of 40 percent, will produce acontinuous gray rosette pattern to the eye of a viewer, so that theparticular color used in printing a hidden image is not discernible. Ofcourse, if the image is printed in a darker tone and at a much greaterdensity than the other components of the field, it would becomediscernible to a viewer.

The steps to be followed in producing a multicolor halftone field areessentially the same as those described above in connection with thesingle color field 10, except that separate negatives are made of eachof the screen pattern components and these negatives are used to etchindividual plates. The individual plates are then used in sequence toprint the separate colors in forming the composite halftone field.

VIEWING APPARATUS Now that the image has been hidden in the halftonefield I0 and resists detection by the eye or photographic analysis, itis of little value unless its presence can be utilized in some way. Onetype of apparatus for readily bringing out the hidden image in thehalftone field 10 is shown in FIG. 5. The essential component of thisapparatus is a periodic screen 12 of approximately the same frequency asthe components of the halftone field 10. A screen of this sort might becut from the large sheet of film which was originally used in producingthe components of the halftone field. The lines of apertures 2 on thedetecting screen 12 are arranged at the same angle with the upper marginas the lines of dots of the screen 8 which produced the image, in thepresent case, at 105". When the detecting screen 12 is held inregistration over the halftone field l0 and moved across the field, 10as shown in FIG. 6, the resulting light interference magnificationeffects will bring out a blinking image of the letters OK to the eye ofa viewer.

While it will be seen that pictures containing hidden images may be usedin combination with the detecting screen 12 in many diverseapplications, such as in games, advertising displays and articlemarking, a particularly useful and preferred application is in theprinting of security documents to resist unauthorized duplication orcounterfeiting. The apparatus shown in FIG. 5 is particularly intendedfor use in inspecting security documents printed in the manner of thepresent invention. The apparatus may comprise a light table portion 13,having two fluorescent lights 14 mounted below a translucent plate 15,and a viewing lens 16 with the detecting screen 12 disposed between itand the light table 13. The illumination may also be directed downwardlyupon the document and through the detecting screen overlying thedocument. On the other hand, the illuminated document, whetherilluminated from below or above can be projected through a suitable lenswith projected image viewed through a movable halftone screen of properfrequency and orientation. A person wishing to determine whether adocument, for example, a travellers check 17 presented for payment,contains the hidden image and is therefor valid, would insert the checkinto the apparatus. The check 17 is placed on the surface of thetranslucent plate with its upper edge in abutment with a registrationtab 18, so that the individual halftone components will be arranged attheir given angles with respect to the plate margin. The two lights 14below the plate 15 are then turned on by means of a switch 19,presenting a bright image of the check 17 to a person viewing it throughthe viewing lens 16 disposed above the table 13. The detecting screen 12which will be arranged at the same angle as the image halftone on thecheck 17 is mounted in a holder 20, supported by an arm 21 whoseopposite end is connected to an eccentric wheel 22. The wheel 22 isdriven by an appropriate motor 23. The motor 23 may be operated by theswitch 19 that turns on the lights 14, so that, when the switch 19 isthrown, the detecting screen 12 will be moved back and forth across thecheck 17 on the table 13. If the check 17 has been printed with thehidden image, a person looking through the viewing lens 16 will see ablinking image of the letters OK somewhat as indicated in FIG. 6.Counterfeits will present no image.

COUNTERFEIT PREVENTING MEASURES Alternative apparatus is shown in FIGS.8 and 9 wherein a security document such as a check 30 is placed onplatform 29 having an opaque plate 31 with registration tabs 32.Illumination from lights 33 and 34 above the platform, is directedtoward the document from a 45 angle. In FIG. 8, the moire screen 35 ismounted in a hinged frame 36, to permit the screen to be placed directlyupon the check, the screen being hinged to permit the placing of thecheck in registration, and removal therefrom after examination. In FIG.9, the moire screen 37 is located at a predetermined distance above theplate 32, in parallel relation, and a projection lens 38 projects animage of the check 30 in the plane of the moire screen of the same size.In either apparatus a vibrator to vibrate the plate in a horizontalplane may be provided as is indicated at 39.

It will be realized that while security documents printed in the manneralready discussed would resist duplication by an unknowingcounterfeiter, still one familiar with the techniques of the presentinvention could reproduce documents printed in this manner without toomuch difficulty. For example, documents printed in a single color couldbe duplicated by even an unknowing counterfeiter by photographing thedocument in toto and etching a single plate from the photograph whichwould necessarily contain the image hidden in the halftone field. Whilethis could not be done with a multicolor document, it might be possibleto analyze the hidden image using the detecting screen. A further stepwhich may be employed in the method of the present invention to confoundthe counterfeiter, is the use of a black overprint as shown in FIG. 7. Adocument printed in the manner of the check 17 of FIG. 6 is also printedwith a black halftone screened at 90 with respect to the upper margin.The dot figures 24 of this halftone may be of the same size, butpreferably are much larger than those of the three component anddifierent frequency halftones. A counterfeiter, in attempting tophotograph individual halftone components by screening out the remainingcomponents, in the case of a single color, or filtering them out whendifferent colors are used, must contend with the black overprint. Eachof the component photos, or exposures of a single-color document willcontain parts of the black overprint, as well as the particularcomponent screen pattern so that the resulting plate or plates willproduce a halftone field on the counterfeit document which appears muchdarker or more dense than the field of the original document. If thehalftone field is printed in three colors, and therefore, threephotographs must be made, using appropriate color filters, each of thephotographs will contain an image of the black overprint as the blackcannot be filtered out. The reproduction in this case will generally notonly be darker than the original but will also contain various moireeffects, since each component will contain an image of the blackhalftone in its own color, but screened at a different angle.

A further step to resist counterfeiting of the halftone field is todisplace the component halftones at other than 30 degree angles. Aspreviously mentioned, in halftone printing if the various halftonecomponents are not properly displaced, a moire efiect will occur, whichwill result in a blotching, waviness, or other irregularities in thecomposite halftone field. However, in multicolor printing, it ispossible to vary the displacement of the components somewhat from the 30displacement without encountering the moire effects. When using neutralcolors, it is generally only possible to vary the displacement from 30by plus or minus a half degree, but with the regular colors, thedisplacement may be as much as plus or minus 2 before a moire willappear. Thus, a person attempting to duplicate a halftone field mustprint in exact registration with the original components and if theregistration is just slightly off, a moire pattern will appear, -or thefield will have a somewhat different appearance from the original. Sucha shift in the printing of a duplicate will show up better if theoriginal field is of a heavy density, rather than of a low density.

It is also possible to print a solid color over the halftone field andstill bring out the blinking hidden image using the detecting screen.The solid color may even be the same as that used in printing thehalftone or a solid black; the only requirement being that the solidcolor be of the same or close to the printing density of the hiddenimage background. As long as the inks used are translucent, theinterference magnification effect will lift out" the halftone of thehidden image, rendering it detectable to the eye.

A method of halftone printing is thus presented wherein an image may behidden in a halftone field resisting detection by the eye or a camerabut which may be readily detected by the use of a halftone screenapparatus. The method produces a halftone field which resistsduplication and also embodies the use of a black overprint and varyingof the displacement angle of thehalftone components in the field to foilcounterfeiting. In addition, halftone screens with unusual dotconfigurations and hidden images in unique or complicated form may beused to render duplication practically impossible.

What is claimed is:

1. A method of printing a hidden image in a halftone field comprisingthe steps of:

a. printing an overall halftone at a given angle, the limits of saidoverall halftone defining the limits of said field;

b. printing a second halftone in said field at a different angle fromsaid overall halftone, and containing blank portions in the form of saidimage; and

c. printing a third halftone in the form of said image at an angledifferent from the angles of said overall and second halftones and inregister with said blank portions in said second halftone; two of saidhalftones being printed at approximately thirty degree angles withrespect to the other halftone and all of said halftones being of asimilar density and tone, that is, having a period greater than onehundred lines per inch and being of a similar dot size and degree ofbrilliance and saturation with respect to a gray scale.

2. The method of claim 1 including the step of printing a black patternin said halftone field.

3. The method of claim 2 wherein said black pattern is a halftoneprinted at a difierent angle from said overall, second and thirdhalftones.

tone, that is, having a period greater than one hundred lines per inchand being of a similar dot size and degree of brilliance and saturationwith respect to a gray scale. 8. The article of claim 7 including ablack pattern printed on solid color over said halftone field in atranslucent ink and of a i rf similar reflection density to that of thehalftones.

6. The method of claim 1 wherein said overall, second and thirdhalftones are of different colors and displaced from each other byangles in the range from 28 to 32.

7. An article printed with a hidden image, comprising:

a. a surface on said article for receiving halftone printing;

b. an overall halftone printed at a given angle on said surface;

c. a second halftone printed at a different angle from said overallhalftone on said surface, said second halftone containing blank portionsin the form of said image; and

d. a third halftone in the form of said image printed at an angledifferent from the angles of said overall and second halftones and inregister with said blank portions in said second halftone; two of saidhalftones being printed at approximately angles with respect to theother halftone and all of said halftones being of a similar density and9. The article of claim 8 wherein said black pattern is a halftoneprinted at a different angle from said overall, second and thirdhalftones.

10. The article of claim 9 wherein the black halftone is of much largerdot figures than the overall, second, and third halftones.

11. The article of claim 7 wherein the surface is translucent andincluding a solid color in a translucent ink and of a similar reflectiondensity to that of the halftones printed over said surface.

12. The article of claim 7, wherein said overall, second and thirdhalftones are of different colors and displaced from each other byangles in the range from 28 to 32.

13. The article of claim 7 wherein the article is a security document.

l III l

1. A method of printing a hidden image in a halftone field comprisingthe steps of: a. printing an overall halftone at a given angle, thelimits of said overall halftone defining the limits of said field; b.printing a second halftone in said field at a different angle from saidoverall halftone, and containing blank portions in the form of saidimage; and c. printing a third halftone in the form of said image at anangle different from the angles of said overall and second halftones andin register with said blank portions in said second halftone; two ofsaid halftones being printed at approximately thirty degree angles withrespect to the other halftone and all of said halftones being of asimilar density and tone, that is, having a period greater than onehundred lines per inch and being of a similar dot size and degree ofbrilliance and saturation with respect to a gray scale.
 2. The method ofclaim 1 including the step of printing a black pattern in said halftonefield.
 3. The method of claim 2 wherein said black pattern is a halftoneprinted at a different angle from said overall, second and thirdhalftones.
 4. The method of claim 3 wherein the black halftone is ofmuch larger dot figures than the overall second and third halftones. 5.The method of claim 1, including the step of printing a solid color oversaid halftone field in a translucent ink and of a similar reflectiondensity to that of the halftones.
 6. The method of claim 1 wherein saidoverall, second and third halftones are of different colors anddisplaced from each other by angles in the range from 28 to 32*.
 7. Anarticle printed with a hidden image, comprising: a. a surface on saidarticle for receiving hAlftone printing; b. an overall halftone printedat a given angle on said surface; c. a second halftone printed at adifferent angle from said overall halftone on said surface, said secondhalftone containing blank portions in the form of said image; and d. athird halftone in the form of said image printed at an angle differentfrom the angles of said overall and second halftones and in registerwith said blank portions in said second halftone; two of said halftonesbeing printed at approximately 30* angles with respect to the otherhalftone and all of said halftones being of a similar density and tone,that is, having a period greater than one hundred lines per inch andbeing of a similar dot size and degree of brilliance and saturation withrespect to a gray scale.
 8. The article of claim 7 including a blackpattern printed on said surface.
 9. The article of claim 8 wherein saidblack pattern is a halftone printed at a different angle from saidoverall, second and third halftones.
 10. The article of claim 9 whereinthe black halftone is of much larger dot figures than the overall,second, and third halftones.
 11. The article of claim 7 wherein thesurface is translucent and including a solid color in a translucent inkand of a similar reflection density to that of the halftones printedover said surface.
 12. The article of claim 7, wherein said overall,second and third halftones are of different colors and displaced fromeach other by angles in the range from 28 to 32*.
 13. The article ofclaim 7 wherein the article is a security document.